Apparatus for effecting the growth of crystals in vacuo



Patented Mar. 24, 1936 UNITED STATES APPARATUS FOR EFFECTING THE GROWTH OF. CRYSTALS IN VACUO Hans Christian August Behrens, Duvenstedt, Post Wohldori', near Hamburg,

Germany, assignor,

by mesne assignments, to firm Rud. Otto Meyer,

Hamburg, Germany Application September 28, 1933, Serial No. 691,407

, In Germany October 29, 1932 6 Claims. (Cl. 159-45) when effecting the separation of salts from solutions it is possible, according to the time taken, to produce a more or less coarse salt granule. The time necessary for producing a granule of given size is governed by the size of the apparatus, e. g. crystallization vessels, to be used. In the usual method, one particularly used in the potash industry, hot saturated liquors are supplied to vats wherein they are cooled down partly as a result of radiation and partly owing to evaporation, with the consequence that salt crystals form chiefly on the surface, said crystals consisting of many individual crystals of acceptable size and sinking to the bottom upon attaming a certain weight. The advantage of this cooling method consisted in the production of a coarse salt granule; the chief disadvantages lie in the unusually long time required (2 to 4 days) and the complete loss of the heat employed for effecting the dissolution. The salt solution can be more rapidly cooled down with the assistance of cooling coils, which withdraw the heat from the salt solution to be cooled with the aid of water, or the solvent liquor which is to be heated up again, or some other cooling agent. In order to prevent deposition on the cooling coils as far as possible, the solution has been agitated by means of suitable stirring mechanism. In order to produce a granule which is to some extent coarse, the stirring mechanism must be so constructed that it continually brings up to a certain percentage of the crystals forming to the surface again, where, growing further, they gradually sink to the bottom again. Apart from the large power requirements, in this cooling method the cooling elements are subjected to great wear.

In recent times the solutions have frequently been cooled down by partial evaporation of the solvent in vacuo. If the solution is cooled down suddenly over a certain temperature zone an extraordinarily fine crystal granule (crystal meal) is obtained, which does not conform to the user's wishes owing to the difficulties in working it up. To obtain by vacuum cooling at granule which is to a certain extent coarse it is necessary (I) To agitate the solution violently, so that the crystal particles are held in suspension,

(II) To make the appliances used of a certain size so that suiiicient liquor, or crystals, remain in the vacuum chamber a. definite time, and

(III) To eiiect the cooling of the solution in the apparatus divided up into individual stages each time only to such an extent that a certain temperature interval is not exceeded.

For example, in the case of Epsom salt, in the evaporation of the saturated liquor cooling must not be effected suddenly under 5 0., in one stage but this temperature interval must be maintained as a maximum, and further the solution must be given the requisite time necessary for the completion of the crystallization process. If a solution has to be cooled down from 50 to 20 then, in the case of Epsom salt for example, cooling must be effected six times to the extent of about 5 each time. In order not to have to provide six sets of apparatus, operating under different vacua corresponding to the temperature each time coming into question, so-called multiple chamber cooling apparatus have been installed. These latter mostly consist of a cylindrical vessel subdivided into a plurality of chambers by separating walls. In this multiple chamber apparatus contrived thus the liquor is led over into the other chamber through slots situated at the bottom of the separating sheet wall in question. Since difierent absolute pressures prevail in the individual chambers a varying liquor level is set up. The difference in level from chamber to chamber is practically equal to the pressure difference of the saturated vapour pressures. It follows, that very little liquor is present in the first chamber, whilst more is in the second chamber and most in the third chamber. 'Owing to the varying liquid level the device is utilized only to a small percentage extent, since, as previously described, it is essential to subject the liquor to the vacuum for a certain time in order to complete the crystallization. In order to produce a uniform-granule the first chamber must be made correspondingly longer, so that it has the same holding capabilities as the third chamber with its higher liquor level.

According to the present invention the crystal growth is improved by considerably enlarging the holding space of the bodies, this being effected not by arranging any slots in the separating walls but by using pipes or equalization chambers for transferring the liquor from one stage to the other, said pipes or chambers being, for example, inserted in the separating walls. Each chamber is consequently completely closed off in itself and the liquor, in order that it can pass over into the other chamber, must ascend to the height of the over-flow pipe of the separating wall of the equalization chamber. The 'pressure difierence as compared with the subsequent stages always adlusts itself in the pipe or the equalization chamber, and in the over-flow pipe the liquoris at a lower level than in the appertaining evaporation stage.

In the accompanying drawing one constructional form of the three-chamber apparatus is shown schematically by way of example.

Fig. 1 is a longitudinal section through a multichamber evaporator with overflow pipes.

Fig. 2 is a cross-section along line 2-2. of

Fig. 3 is a longitudinal section through a multichamber evaporator with overflow conduits.

Fig. 4 is a cross-section of Fig. 3, and

Fig. 5 a cross-section taken along line 5-5 of Fig. 1.

In Fig. 1 over-flow pipes (5 and 6) are provided, whilst in Fig. 3 these are replaced by overflow conduits (l5 and I6). These are formed by double partitions of which 2 and 3 extend from the top of the container to near the bottom thereof, while l1, l8 extend from said bottom up to the level of the solution.

The apparatus I is divided into three or more parts or chambers by the separating walls or partitions 2 and 3. (Formerly these walls were provided at the bottom with slots 4, shown in dotted lines.)

According to the invention pipes 5 and 6 are arranged which efiect communication from a previous chamber to a subsequent chamber (Fig. 1). The liquor passes through the inlet 1 into the first part or chamber of the evaporator, whence it passes through the pipe 5 downwards into the second part or chamber, and through the pipe 6 into the third part or chamber, in order to pass into another chamber, or to be finally led away, through the pipe 8.

If the temperature of the liquor amounts in the first part or chamber of the evaporator to 50, in the second part or chamber to 45, and in the third part or chamber to 40, then in the pipe 5 there is a difference in head of about 350 mm. water column, and in pipe 6 one of 300 mm. water column. As a result of this difference, amounting to altogether 650 mm. water column, more liquor may be accumulated or stacked up in the device I as compared with the older constructions. Consequently the time for which the liquor is maintained in the vacuum chamber is increased and as a result the crystal granule also is increased. In the present example the new arrangement permits of a 40% higher liquor accumulation or stacking. If the salt granule obtained in the older plants is satisfactory new plants can be constructed with a containing space which is about 40% smaller, as a result of which the costs of the plant are considerably diminished. If solutions of are being cooled down the advantage of the present arrangement is much greater still, for then the temperature difierence of 5 C. corresponds to a much higher pressure fall. The solution is preferably continuously agi tated by means of a stirring device, e. g. stirring paddles 9, l0 and II. These paddles are driven by, for example, the driving shaft l2 which is actuated by the motor l3 or through the drive It. Owing to the uniform level of the liquid in the individual chambers, a less violent stirring sufiices. The device shown in Fig. 2 operates in the same way as that shown in Fig. 1; in it the pipes 5, 6 are replaced by the equalization conduits l5, [6, which function in the same way.

It should be noted that the liquid level in each compartment differs as indicated by reference characters a, b and c in Figures 1 and 3.

What I claim is:

1. An apparatus for crystallizing salts from solutions by partial evaporation of the solvent under reduced pressure, consisting of a container, upright partitions in said container subdividing the container intoa plurality of compartments, a charging opening for the solution near the bottom of one of the end compartments, a discharge opening for the solution near the top of the other end compartment, and in each compartment, except that with the discharge opening, a conduit adapted to establish communication between two adjacent compartments, the upper end of said conduit being substantially level with said discharge opening and forming the exit for the solution from a previous compartment, and the opening at the lower end providing an inlet into a subsequent compartment near the bottom thereof so that solution leaves one compartment .for the next only when its level rises sufliciently to cause it to overflow into the conduit in said compartment, whereby a difierent pressure can be maintained in each compartment.

2. An apparatus for crystallizing salts from solutions by partial evaporation of the solvent under reduced pressure, consisting of a container,

upright partitions in said container subdividing the container into a plurality of compartments, each partition having an opening near the bottom providing communication between said compartments, a charging opening for the solution near the bottom of one of the end compartments, a discharge opening for the solution in the wall of the other end compartment, and in each compartment, except that with the discharge opening, an additional wall extending upwardly from the bottom of the container substantially to the level with said discharge opening near, but spaced from said partition, whereby a conduit is formed, so that solution leaves one compartment for the next only when its level rises sufiiciently to cause it to overflow into said conduit, whereby a different pressure can be maintained in each chamber.

3. An apparatus for crystallizing salts from solutions by partial evaporation of the solvent under reduced pressure, comprising several chambers with stirring devices in each chamber, a charging opening for the salt liquor being pro" vided near the bottom of one end chamber and a discharge opening for the salt solution and the crystals obtained being provided in the wall of the ultimate end chamber at the desired height of the solution in the container and conduits arranged between adjacent chambers for the overflow of the liquor and the crystals, said conduits each having an inlet arranged at about the same level as said discharge opening and each conduit having an outlet extending into the adjacent chamber below the outlets of the said chamber.

4. An apparatus for crystallizing salts from solutions by partial evaporation of the solvent under reduced pressure, comprising several chambers with stirring devices in each chamber, a charging opening for the salt liquor being provided near the bottom of one end chamber and a common discharge opening for the salt solution and the crystals obtained being provided in the wall of the ultimate end chamber at the desired height of the solution in the container and conduits arranged between adjacent chambers for the overflow of the liquor and the crystals, said conduits comprising pairs of vertical partitions arranged for determining a substantially common level of the liquid in the several chambers, one of said partitions in each pair rising from the bottom of each chamber to said height of the solution and terminating substantially level with said common discharge opening while the other partition of said pair oi. partitions extends from the top of each chamber but ends before reaching the bottom of the chamber thereby shuttin off the air pressure so that each pair of partitions forms a conduit wherein diflerent liquid levels can be maintained.

5. An apparatus for crystallizing salts from solutions by partial evaporation of the solvent under reduced pressure, comprising several chambers with stirring devices in each chamber, the inlet for the salt liquor being provided near the bottom of an end chamber and only one outlet for the salt liquor and the crystals obtained being provided in the wall of the end chamber most remotely situated from the first chamber and communicating pipes arranged between the chambers which begin at the desired liquid level in one chamber and end near the bottom of an lutions by partial evaporation of the solvent under reduced pressure, comprising several chambers with stirring devices in each chamber, the

inlet for the salt liquor being provided near the bottom of an end chamber and only one outlet for the salt liquor and the crystals obtained being provided in the wall of the end chamber most remotely situated from the first chamber and for the communication of the chambers with each other, vertical separating walls are arranged for determining the height of the liquid in the chambers said separating walls rising from the bottom of the chambers to about the same height and terminating substantially level with said outlet opening while other separating walls extending from the upper part of the chamber but ending before reaching the bottom of the chamber are provided for shutting ofi the air pressure so that both separating walls form channels wherein different liquid levels can be maintained.

HANS CHRISTIAN AUGUST BEHRENS. 

